Light-sensitive switching display device

ABSTRACT

An electroluminescent storage and readout device in which a light-responsive switch is utilized to control an electroluminescent material and in which the light-responsive switch is controlled by a light-generating information system. A high-voltage storage/readout device is optically coupled to a low-voltage electroluminescent drive panel in a two-stage device.

United States Patent LIGHT-SENSITIVE SWITCHING DISPLAY DEVICE 5 Claims,7 Drawing Figs.

U.S.Cl 250/213,

Int. Cl 11011 17/00,

Field of Search 250/213,

[56] References Cited UNITED STATES PATENTS 2,920,232 l/1960 Evans250/213 2,989,641 6/1961 Nicoll 250/213 2,988,647 6/1961 Duinker et al..313/108 3,059,118 10/1962 Koury 250/213 3,304,430 2/1967 Biard et a1.250/217 Primary ExaminerWalter Stolwein Attorneys- F. H. Henson and C.F. Renz ABSTRACT: An electroluminescent storage and readout device inwhich 'a light-responsive switch is utilized to control anelectroluminescent material and in which the light-responsive switch iscontrolled by a light-generating information system. A high voltagestorage/readout device is optically coupled to a low-voltageelectroluminescent drive panel in a two-stage device.

, 2e 2e 24 2e LIGHT-SENSITIVE SWITCHING DISPLAY DEVICE BACKGROUND OF THEINVENTION This invention relates to electroluminescent display systemsand in particular to electroluminescent storage/readout devices in whicheach cell is combined with a switch activated by light.

A problem is encountered in connecting a high-voltage storage/readoutdevice electrically to an information system which is generally composedof transistors and microcircuitry. The basic problem is to connect ahigh-voltage, high-current electrical device to low-voltage, low-currentelectrical information system. This problem has been overcome in theprior art by electrically isolating but optically coupling thestorage/readout device to a controlling light-pulsing device which ispart of the information system. The light-pulsing device can be alow-voltage, low-output, electroluminescent device, which is compatiblewith microcircuitry. Such an electroluminescent pulsing device usuallyhas a short light-hour life but since it is only pulsed and does notoperate continuously for long periods of time a short light-hour life isnot deleterious.

The prior art devices that overcome this problem are all three-stagedevices, a light-pulsing stage, a storage stage and a readout stage. Thepresent invention simplifies this device into basically a two-stagedevice. Previously the storage portion was an electroluminescent layerthat emitted light to latch on a photoconductive switch. The presentinvention accomplishes storage by utilizing a photoconductive switchwhich is designed to operate as a bistable switch.

Hysteretic material such as cadmium selenide has a current voltagecharacteristic which causes its impedance to suddenly decrease when thevoltage gradient applied across the material exceeds a certain value.The voltage must then be decreased considerably below this value for aminimum period of time before the material will resume its formerhigh-impedance state. This hysteretic characteristic of cadmium selenidehas been used for electroluminescent switching but always with voltagepulse switching.

SUMMARY OF THE INVENTION This invention relates to the use of thecharacteristic that cadmium selenide, when used as a hysteretic materialand electrically biased at a voltage just less than that required toswitch to a low-impedance state can be switched from a state of highimpedance to a state of low impedance by application of a suitable lightpulse. This is accomplished when a hysteretic photoconductor with animpedance characteristic such as cadmium selenide is biased with asinusoidal voltage such that it has two stable conditions, alow-impedance state or on" position and a high-impedance state or offposition. The physical embodiment of the invention hereinafter describedin a device comprising two panels, an electroluminescent panel combinedwith cadmium selenide as a hysteretic photoconductive switching elementtriggered by light pulses generated by a low-voltage electroluminescentdrive panel. The latter is x-Y addressed and is electrically isolatedbut optically coupled to the electroluminescent hystereticstorage/readout panel.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and applications willbecome obvious from the following description and the drawings in which:

FIG. I is a graph of the voltage current characteristic of a hystereticphotoconductive material such as cadmium selenide when switched by lightpulsing.

FIG. 2 represents a sectional view of one embodiment of the invention, atwo-panel construction, hysteretic optically switched electroluminescentdevice.

FIG. 3 represents a cutaway plan view of the device shown in the upperpart of FIG. 2.

FIG. 4 represents a plan view of the illuminating layer shown in FIGS. 2and 3.

FIG. 5 represents a sectional view of a second embodiment of theinvention, a single-panel-constructed, hysteretic photoconductivelyswitched electroluminescent device.

FIG. 6 represents a cutaway plan view of the device shown in FIG. 5.

FIG. 7 represents a plan view of the illuminating layer also shown inFIGS. 5 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, where the horizontalordinate is voltage and the vertical ordinate is current on alogarithmic base,t he line from the origin 0 to point a represents the0, state, while the line from f to b represents the low-impedance state.It requires a voltage pulse greater than V or a light pulse of anappropriate duration and intensity combined with a suitable biasingvoltage V,,, where V is greater than V, and less than V, to switch fromthe high-impedance state to the low-impedance state. The hystereticmaterial will revert to its former state of highimpedance if the voltagedrops to zero for a specified time. If the hysteretic material isnormally biased between V, and V a suitable light pulse will switch thematerial to the low-impedance state and a suitable negative voltagepulse will switch the hysteretic material back to its formerhigh-impedance state.

FIG. 2 represents a cross-sectional view of a hysteretic opticallyswitched electroluminescent device 30 comprising two separate panels,storage/readout panel 20 and drive panel 10, sealed between transparentfront cover 32 and opaque back cover 31. These panels are electricallyinsulated but optically coupled by a light-transmitting mediumcomprising a transparent substrate 11, which does not diffuse lightreadily, thereby minimizing crosstalk.

The drive panel 10 is a known, prior art, x-Y-addressedelectroluminescent panel. It comprises a transparent substrate 11;electroluminescent layer 12, linear X-electrodes 15 and transparentlinear Y-electrodes l6 mutually at right angles. A voltage-pulsingsource 35 supplies voltage pulses to drive panel 10 by selectivelyenergizing appropriate X- and Y-electrodes to excite the chosen elementat the crossing of the X- and Y-electrodes.

The storage/readout panel 20 comprises; a main substrate 21; an array ofrear terminal pads 22, readout terminal pads 23 and through holeconductors 24; hole filler 25; general terminal 26; hysteretic material27; electroluminescent readout layer 28; and transparent electrode 29.Electrically coupled to the storage/readout panel 20 is the voltagesource 33 which applies a sinusoidal bias potential across the seriescombination of the hysteretic material and electroluminescent layer andcan further provide negative voltage pulses when required to erase theinformation stored in the memory of the storage/readout panel 20.

FIG. 3 shows a layer-by-layer cutaway plan view of the device shown inFIG. 2. The outermost layer at the upper left is the transparent frontcover 32 on the readout side of the storage/readout panel. Directlybelow that is the transparent electrode layer 29 then comes theelectroluminescent readout layer 28. The next three sections representthe component parts of the electrically floating electrodes eachconsisting of a rear terminal pad 22, and a readout terminal pad 23joined by a through hole conductor 24. Also shown in the three sectionsare, main substrate 21, common terminal 26, hysteretic material 27 andinert hole filler 25.

FIG. 4 is a plan view of the readout terminal pads 23 as they wouldappear when illuminated. Other parts designated in FIG. 4 are, throughhole conductors 24, hole filler 25, and substrate 21.

FIG. 5, a preferred embodiment of the invention, is anelectroluminescent device 60 constructed around a single main substrate61. The drive panel and storage/readout panel are functionally but notphysically seperable. Electroluminescent device 60 comprises:electroluminescent trigger layer 12; X-Y-electrode grid consisting ofX-electrodes l and transparent Y-electrodes 16; floating electrodes eachconsisting of a solid through hole conductor 67 and a readout terminalpad 23; common terminal 65; hysteretic material 27 readoutelectroluminescent layer 28; transparent electrode 29; opaque rear cover31; transparent front cover 32; main substrate 61 and opaque insulatinglayer 66. Also shown in FIG. 5 is voltage-pulsing source 35 and voltagesource 33. Voltage-pulsing source 35 is functionally connected acrossthe drive electroluminescent panel which is formed in strata on thefirst surface of the main substrate 61. Voltage source 33 isfunctionally connected across the storage/readout panel which is formedin strata on the second surface of the main substrate 61. Thestorage/readout panel and the drive panel are optically coupled throughand electrically isolated by main substrate 61.

FIG. 6 is a cutaway sectional plan view of the storage/readout portionof the device 60. Starting from the upper left-hand corner are thefollowing: transparent front cover 32; transparent front electrode 29;readout electroluminescent layer 28; readout terminal pads 23; opaque,low dielectric constant, epoxy lever 66, through hole conductors 67;hysteretic material 27; common terminal 65. and main substrate 61.

FIG. 7, similar to FIG. 4, is a plan view of the illuminated readoutterminal pads 23, separated by opaque epoxy 66. Hidden, below theterminal pads 23, in dotted lines is the through hole conductors 67.

Voltage source 33 is electrically connected between the fronttransparent electrode 29 and the common terminal, numbered 26in device30 and numbered 65 in device 60.

The hysteretic material 27, see FIGS. 2, 3, 5 and 6; in theelectroluminescent device is isolated from all exterior ambient light byopaque back cover 31, the opaque floating electrodes 23 and commonterminal 26, shown in FIGS. 3 and 4 for device 30. In device 60 thehysteretic material is similarly isolated from exterior light by theopaque rear cover 31, the opaque floating electrodes 23, and the opaquelayer 66.

Information to be displayed is sequentially read into addressedpositions in the X-Y grid with voltage pulses. These voltage pulses aretransformed into light pulses by the drive electroluminescent layer 12.The light pulses from the drive electroluminescent layer impinge upon aparticular section of the continuous layer of electrically biasedhysteretic material which shall be called a segment of the hystereticmaterial. The segment switches from its normal state of high impedanceto a state of low impedance. This change of impedance redistributes thevoltage between the segment of the hysteretic material and a segment ofthe storage/readout electroluminescent layer such that the segment ofthe electroluminescent layer luminesces. If the biasing voltage on thehysteretic layer is maintained the display on the storage/readout devicewill remain luminescent even though the light pulse from the drivedevice has ended. To erase the information displayed, a suitablenegative voltage pulse is provided by voltage source 33 through thestorage/readout device and switches all segments of the hystereticmaterial back to their normal state of high impedance, redistributingthe voltage between the segments of hysteretic material and the segmentsof the storage/readout electroluminescent layer such that no segments ofthe storage/readout electroluminescent layer will continue to luminesce.Information will normally be changed by erasing all the presentinformation and reinserting the updated information by sequentialvoltage pulsing of the drive device.

In determining the sinusoidal biasing voltage for the hystereticmaterial from the hysteretic curve of FIG. I the voltage divisionbetween the hysteretic material and the storage/readoutelectroluminescent layer in both the low-impedance and high-impedancestate must be considered. The impedances of the electroluminescent andhysteretic elements must be such that in high-impedance state thevoltage across the hysteretic material is less than V,, (FIG. I). Inlow-impedance state the voltage division must be such that the voltageacross a hysteretic material is maintained greater than V (FIG. 1). Thusthe impedance of the hysteretic material is matched with that of theelectroluminescent layer so that they hysteretic material will act as abistable switch.

In addition to this criterion the impedance must be matched such thatthe device will not fail due to power overload after a limited period ofuse. This may occur when the hysteretic material dissipates an excessiveamount of power thus causing the materials characteristic to deterioratequickly. Prior to manufacturing, a single switching element of thehysteretic material should be tested in conjunction with a singleelement of the electroluminescent readout layer in order to establish areasonable life. This is accomplished by varying the impedance of theelectroluminescent readout segment and the impedance of the hystereticswitching material within the previously established limits and testingto obtain maximum life. Theimpedance of the hysteretic switching elementcan be varied by varying the bulk parameters in the hysteretic switchitself. Included in these bulk parameters are the physical and geometricparameters associated therewith, these parameters determine the criticalpower dissipation per switch and thereby the life of the device.

The device 60 (FIG. 5) is manufactured by the successive deposition oflayers onto the two major surfaces of the transparent main substrate 61.On the second surface the following layers are deposited in theirrespective order: the common terminal layer 65; the continuous layer-ofhysteretic material 27; the low dielectric constant, insulative, opaquelayer 66 (a black epoxy) leaving predetermined separate circularportions of the hysteretic material exposed, to be covered with aluminumdeposits which form the solid through hold connections 67; the aluminumreadout electrode pattern comprising many readout terminal pads 23 (FIG.7). Onto the combined aluminum and opaque layer, the continuouselectroluminescent layer 28, followed by an optically transparent goldelectrode layer 29 forming the transparent electrode and also anelectrical connection to the high-voltage supply 33.

The following layers are deposited onto the first surface of the mainsubstrate 61 in this order: the transparent Y-electrode grid 16, thecontinuous drive electroluminescent layer 12, and the X-electrode grid15. Lastly the complete device 60 is sealed between a front and backcover with the necessary electrical connections brought through orbetween the sealing cover members.

For a more detailed description of the materials used and the methods ofmanufacturing similar devices, refer to copending US. Pat. applicationSer. No. 671,273, filed Sept. 28, I961, entitled ElectroluminescentDevice by R. E. Lake et al. and assigned to the assignee of the presentinvention.

In the description of the drawings, the word panel has been used sinceit most aptly describes the geometry of the particular embodiment shownin the drawings, but the invention is broader and is not subject to suchgeometric limitations. Thus an example of a simple embodiment of thisinvention is a single cell, off/on, display or indicator utilizinghysteretic material as a bistable switch activated by light pulses, suchas one segment or unit of the display as described or illustrated in thedrawings.

, I claim as my invention:

1. An electroluminescent device comprising: a storage/readout sectionand a light-pulsing trigger section, said trigger section comprising anx-Y-addressed electroluminescent panel and electrically isolated frombut optically coupled to said storage/readout section, saidstorage/readout section including an electroluminescent layer controlledby a light responsive switch, voltage means to bias said switch suchthat at least a portion of said readout/storage section has two stableconditions, an on" condition and off condition, said switch activatingsaid portion of said readout/storage section to the on" condition uponthe occurrence of a light pulse from said trigger section impinging onsaid switch and deactivating said readout/storage section to the off"condition by reducing the bias from said voltage means and an opaquelayer to prevent light other than said light pulses from actuating saidswitch. v

2. An electroluminescent device comprising two sections and two covers;a storage/readout section and a light-pulsing trigger section; saidstorage/readout section including the following: a hysteretic materiallayer, opaque floating electrodes, an electroluminescent layer, atransparent front electrode, a main substrate, an opaque commonterminal, said electroluminescent layer and said hysteretic materiallayer being electrically connected in series by said floatingelectrodes, said floating electrodes and said common terminaloverlapping one another in parallel planes so that no light from saidelectroluminescent layers impinges upon the hysteretic material, saidtrigger section comprising an xY-addressed electroluminescent paneloptically coupled to but electrically isolated from said hystereticmaterial, said first cover being a transparent light-pulsing andcovering said storage/readout section, said second cover being an opaqueinsulator and covering the rear of said trigger panel.

3. An electroluminescent device as claimed in claim 2 including, voltagemeans to bias said hysteretic material such that at least an element ofsaid electroluminescent layer has two stable conditions, an on conditionand an off c0ndition, said hysteretic material layer being actuated bylight pulses from said trigger section and in turn activating saidelement to said on condition.

4. An electroluminescent device comprising two panels stratified on amain substrate, said substrate having a first and second major surface,said substrate being an electrical insulator and a light-transmittingmedium; a first panel being a trigger panel including an X-Y-addressedelectroluminescent layer on the second surface of said main substrate, asecond panel, being a storage/readout panel, provided on said firstsurface; said second panel comprising the following layers:

a conductive common terminal layer, a hysteretic material layer, anopaque layer, a floating electrode layer consisting of many separatefloating electrodes, an electroluminescent layer electrically connectedin series with said hysteretic material, a transparent electrode layer,said device sealed by a front transparent cover over said second paneland opaque rear cover over said first panel, said opaque layer inconjunction with said floating electrode layer optically isolating saidhysteretic material from said electroluminescent layer and from ambientlight entering through said front transparent cover, means toelectrically bias the series combination of said electroluminescentlayer and said hysteretic material such that said combination has twostable conditions and is activated from a first condition to a secondcondition by light pulses from said trigger panel. 5. Anelectroluminescent device as claimed in claim 4 wherein said opaquelayer is an insulative layer having a low dielectric constant.

1. An electroluminescent device comprising: a storage/readout sectionand a light-pulsing trigger section, said trigger section comprising anx-Y-addressed electroluminescent panel and electrically isolated frombut optically coupled to said storage/readout section, saidstorage/readout section including an electroluminescent layer controlledby a light responsive switch, voltage means to bias said switch suchthat at least a portion of said readout/storage section has two stableconditions, an '''' on'''' condition and '''' off'''' condition, saidswitch activating said portion of said readout/storage section to the'''' on'''' condition upon the occurrence of a light pulse from saidtrigger section impinging on said switch and deactivating saidreadout/storage section to the '''' off'''' condition by reducing thebias from said voltage means and an opaque layer to prevent light otherthan said light pulses from actuating said switch.
 2. Anelectroluminescent device comprising two sections and two covers; astorage/readout section and a light-pulsing trigger section; saidstorage/readout section including the following: a hysteretic materiallayer, opaque floating electrodes, an electroluminescent layer, atransparent front electrode, a main substrate, an opaque commonterminal, said electroluminescent layer and said hysteretic materiallayer being electrically connected in series by said floatingelectrodes, said floating electrodes and said common terminaloverlapping one another in parallel planes so that no light from saidelectroluminescent layers impinges upon the hysteretic material, saidtrigger section comprising an x-Y-addressed electroluminescent paneloptically coupled to but electrically isolated from said hystereticmaterial, said first cover being a transparent light-pulsing andcovering said storage/readout section, said second cover being an opaqueinsulator and covering the rear of said trigger panel.
 3. Anelectroluminescent device as claimed in claim 2 including, voltage meansto bias said hysteretic material such that at least an element of saidelectroluminescent layer has two stable conditions, an '''' on''''condition and an '''' off'''' condition, said hysteretic material layerbeing actuated by light pulses from said trigger section and in turnactivating said element to said '''' on'''' condition.
 4. Anelectroluminescent device comprising two panels stratified on a mainsubstrate, said substrate having a first and second major surface, saidsubstrate being an electrical insulator and a light-transmitting medium;a first panel being a trigger panel including an X-Y-addressedelectroluminescent layer on the second surface of said main substrate, asecond panel, being a storage/readout panel, provided on said firstsurface; said second panel comprising the following layers: a conductivecommon terminal layer, a hysteretic material layer, an opaque layer, afloating electrode layer consisting of many separate floatingelectrodes, an electroluminescent layer electrically connected in serieswith said hysteretic material, a transparent electrode layer, saiddevice sealed by a front transparent cover over said second panel andopaque rear cover over said first panel, said opaque layer inconjunction with said floating electrode layer optically isolating saidhystereTic material from said electroluminescent layer and from ambientlight entering through said front transparent cover, means toelectrically bias the series combination of said electroluminescentlayer and said hysteretic material such that said combination has twostable conditions and is activated from a first condition to a secondcondition by light pulses from said trigger panel.
 5. Anelectroluminescent device as claimed in claim 4 wherein said opaquelayer is an insulative layer having a low dielectric constant.